This invention relates in general to antennas for radiating electromagnetic signals. More particularly, the invention relates to helical antennas for portable radios and other communications equipment.
In the past, relatively large antennas such as the half wave dipole depicted in FIG. 1A were quite acceptable as antennas for low frequency fixed station transceivers. Such half-wave dipole antennas typically exhibit a reasonably broad bandwidth, as illustrated in the return loss vs. frequency graph of FIG. 1B. Unfortunately, if used on a hand-held portable radio, such a half-wave dipole is generally relatively large with respect to the size of the portable radio. The large size of such a dipole antenna often makes it undesirable for portable radio applications.
One solution to the above antenna size problem is to form each of the two quarter wave (.lambda./4) elements of the antenna of FIG. 1A into respective helices thus resulting in the helical antenna of FIG. 2A. Each helical element thus formed occupies significantly less space (.lambda.'/4) than the corresponding element of the dipole of FIG. 1A, but desirably exhibits the same effective electrical length. Although such a helical antenna does result in a decrease in the effective height of the antenna structure employed on a portable radio, the usable bandwidth of the antenna is significantly less than that of the dipole antenna of FIG. 1A. This reduction of usable bandwidth is readily seen in the return loss vs. frequency graph of FIG. 2B for the antenna of FIG. 2A. Moreover, FIG. 3 shows a Smith Chart of the driving point impedance of the antenna of FIG. 2A which demonstrates the narrow banded nature of such a helical antenna.
Those skilled in the antenna arts appreciate that helical antennas generally exhibit a narrow bandwidth. This causes a problem when a particular portable radio is to operate over a relatively wide band of frequencies. For example, to cover the VHF band between 136 and 174 MHz, three or more conventional helical antennas cut to different frequencies must often be used.